The present invention is directed to transistor circuits that provide a differential signal, such as emitter coupled logic (ECL) circuit configurations that are employed as logic gate and driver circuits.
Transistor circuits of the type to which the present invention is directed are typically employed to produce low and high level voltage output signals in response to one or more input signals. For example, they might be used as logic gates or as driver circuits to control output loads. In the operation of these types of circuits, a load capacitance connected to an output terminal of the circuit is charged from a lower output voltage level to a higher output voltage level by an emitter follower transistor. When the input signal changes state, current sinks or pull-down resistors are required to discharge the load capacitances from the higher output voltage level to the lower output voltage level.
The emitter follower transistor is controlled by a circuit which produces a differential output signal. For example, an emitter coupled logic (ECL) circuit can be employed to control the voltage which is applied to the base of the emitter follower transistor. In both the steady and transient states, the standard ECL circuit dissipates a significant amount of power. In addition, the presence of the current sink at the output terminal of the gate diverts driving current from the load capacitance. It is difficult to implement the sink with a bipolar npn transistor in an attempt to alleviate this problem. This difficulty arises from the fact that special circuitry is required to avoid saturation of the transistor and the excessive delays caused thereby.